link to SISO schemas page

specifications/README.md

@@ -12,6 +12,7 @@ space exploration and medicine.*
 * Wikipedia: [Distributed Interactive Simulation (DIS)](https://en.wikipedia.org/wiki/Distributed_Interactive_Simulation) IEEE Standard and [RPR FOM](https://en.wikipedia.org/wiki/RPR_FOM)
 * [Simulation Interoperability Standards Organization (SISO)](https://www.sisostds.org): "Simulation Interoperability and Reuse through Standards"
 * SISO [DIS / RPR FOM PSG - Distributed Interactive Simulation / Real-time Platform Reference Federation Object Model](https://www.sisostds.org/StandardsActivities/SupportGroups/DISRPRFOMPSG.aspx) Product Support Group (PSG)
+* SISO [Standards Data Files](https://www.sisostds.org/Schemas.aspx) including various schemas
 
 ## Working-Group Resources